In a conventional optical disc system, to sense the position of the laser beam in relation to the track on the disc, a main laser beam creates a reflection from the disc. The reflection is typically picked up by 4 photo-diode sensors. FIG. 1 is a conceptual diagram illustrating how such a photo-diode configuration is laid out in relation to the track direction. The outputs of the 4 photo-diodes (when the laser beam is focused on the disc) are shown as signals A, B, C and D, respectively.
Referring to FIG. 2, a conventional optical disc system 20 is shown. The system 200 keeps a laser beam 22 focused on the surface of an optical disc 23. A focusing actuator (not shown) is used to move an objective lens 24 upwards and downwards. The vertical position information of the lens 24 in relation to the optical disc 23 (while moving the objective lens 24 in a vertical direction) is important for motion feedback control of the focusing actuator.
Referring to FIG. 3, a diagram illustrating a conventional focus control zone is shown. When the focus point of the laser beam 22 is moving close to the surface of the optical disc 23, an “S-curve” may be seen in the signal FE and a peak may be seen in the signal BS. A zone Z may creates a focus control zone to turn on the lens focus motion controller. The focus control zone controls the focusing actuator to “lock” the objective lens 24 at the vertical position when the focus point of the laser beam 22 is staying on the data surface of the optical disc 23.
With conventional control methods, a high sampling rate controller (not shown) is needed to quickly focus the laser beam 22 on the data surface of the disc 23 which has multiple optical tracks 26a-26n. The vertical motion of the lens 24 is controlled by the focusing actuator. However, such a high sampling rate controller is generally expensive to implement.
Conventional approaches can only detect the vertical position of the laser beam 22 when the focus point of the laser beam 22 is very close to the surface of the disc 23. The vertical position of the laser beam 22 can be detected by using a very high sampling rate on a focusing error signal FE and a beam strength signal BS. A low sampling rate controller cannot be implemented with conventional methods because the low sampling rate controller generally misses focus point timing. Such focus point timing issues are particularly apparent when the disc 23 has a high vertical deviation while rotating at a high speed.
Because low sampling rate controllers cannot be implemented in conventional approaches, high sampling rate controllers have been found acceptable in reliably searching for the focus point of the laser beam 22. In particular, high sampling controllers reliably direct the focus point of the laser beam 22 to the surface of the disc 23. However, high sampling rate controllers are expensive and are generally more complex to implement, particularly in a software solution.
It would be desirable to provide a method and/or apparatus to obtain the lens vertical position information during a focus search motion from the signal FE with a low sampling rate controller. It would also be desirable to (i) quickly detect the vertical position of the focus point of a laser beam in relation with the surface of a disc as the focus point is moved closer to the disc, (ii) implement an inexpensive low sampling rate controller to control the focusing actuator to successfully search a focus point, and/or (iii) focus the laser beam on the surface of the optical disc, even as the disc rotates with a high vertical deviation at a high speed.